1. Field of the Invention
The present invention relates to a method of driving an organic thin-film EL device which has an organic thin-film EL structure and in which pixels are arranged in a matrix.
2. Description of the Prior Art
An example of a conventional organic thin-film EL display device is disclosed in, e.g., Japanese Unexamined Patent Publication No. 6-301355.
FIG. 1 shows an equivalent circuit of matrix driving of organic thin-film EL elements disclosed in Japanese Unexamined Patent Publication No. 6-301355.
In this reference, the organic multilayered thin film including an emission layer is sandwiched between scanning electrodes X.sub.1 to X.sub.n serving as cathodes and data electrodes Y.sub.1 to Y.sub.m serving as anodes. Pixels each having an organic thin-film EL structure are arranged in a matrix. The scanning electrodes X.sub.1 to X.sub.n are scanned, i.e., transistors 7.sub.1 to 7.sub.n are sequentially turned on one by one to sequentially select the unit electrodes of the scanning electrodes X.sub.1 to X.sub.n and to set them at the ground potential. In accordance with this, a current is supplied to a predetermined unit electrode which should be selected from the data electrodes Y.sub.1 to Y.sub.m in accordance with display data. In other words, a predetermined transistor and a predetermined current supply means which should be selected from transistors 11.sub.1 to 11.sub.m and from current supply means 10.sub.1 to 10.sub.m, respectively, in accordance with the display data, are turned off and set in an operative state, respectively. Hence, a forward bias is applied to selected pixels, concerning the selected unit electrodes of both the scanning electrodes and data electrodes, to cause them to emit light. The nonselected unit electrodes of the scanning electrodes X.sub.1 to X.sub.n are set at a power supply potential V.sub.B by pull-up means R.sub.c comprising resistors and the like. The nonselected unit electrodes of the data electrodes Y.sub.1 to Y.sub.m are set at the ground potential by pull-down means R.sub.e comprising resistors and the like. A reverse bias is applied to the nonselected pixels concerning the nonselected unit electrodes of both the scanning electrodes and data electrodes, and a zero bias or a bias equal to or lower than an emission threshold is applied to the nonselected pixels concerning the selected and nonselected unit electrodes. In this manner, crosstalk caused by the semi-excited state of the nonselected pixels is prevented.
In the prior art shown in FIG. 1, for the sake of simplicity, each of the current supply means 10.sub.1 to 10.sub.m is constituted by one transistor. In fact, a higher-precision constant-current circuit is often employed as the current supply means so that a difference in luminance does not occur among pixels due to a voltage drop caused by the interconnection resistance of the scanning electrodes and data electrodes.
The problem of the conventional method of driving an organic thin-film EL display device described above is that the response speed from selection of a pixel to emission of the selected pixel is low.
The reason for this will be described hereinafter.
FIG. 2 is an equivalent circuit diagram of a drive circuit concerning an organic thin-film EL display device and a conventional driving method.
Scanning electrodes X.sub.1 to X.sub.n are connected, through switches 7.sub.1 to 7.sub.n, to ground when they are selected and to a power supply voltage V.sub.B when they are not selected. Data electrodes Y.sub.1 to Y.sub.m are connected, through switches 11.sub.1 to 11.sub.m, to corresponding current supply means 10.sub.1 to 10.sub.m when they are selected and to ground when they are not selected. Each pixel D(x:1 to n, y:1 to m) having an organic thin-film EL structure is indicated by a diode and a parallel capacitance. As an example, a case will be described wherein a certain unit electrode X.sub.i of the scanning electrodes is selected, and in accordance with this a certain unit electrode Y.sub.j of the data electrodes is selected, so that a pixel D(i, j) concerning the both unit electrodes is caused to emit light.
FIG. 3 is a timing chart showing the conventional method of driving an organic thin-film EL display device. FIG. 3 shows the switching operations of the switches 7.sub.i-1, 7.sub.i, 7.sub.i+1, and 11.sub.j of FIG. 2 and a change over time of the potential of each of the unit electrode X.sub.i of the scanning electrodes and of the unit electrode Y.sub.j of the data electrodes caused by these switching operations.
Immediately before a time period t.sub.i during which the unit electrode X.sub.i of the scanning electrodes is selected by the switch 7.sub.i and set at the ground potential, the unit electrode X.sub.i-1 of the scanning electrodes is selected by the switch 7.sub.i-1 and set at the ground potential, or all the scanning electrodes X.sub.1 to X.sub.n are in the nonselected state. Hence, at least the unit electrodes of the (n-1) scanning electrodes are at the power supply potential V.sub.B. At this time, if the unit electrode Y.sub.j of the data electrodes is not selected by the switch 11.sub.j, as indicated by a solid line, the unit electrode Y.sub.j of the data electrodes is at the ground potential, so that a reverse bias is applied to at least (n-1) pixels of pixels D(1, j) to D(n, j) concerning the scanning electrodes X.sub.1 and X.sub.n and the unit electrode Y.sub.j of the data electrodes, and that the respective parallel capacitances of these (n-1) pixels are charged in the reverse bias direction. Thereafter, during the time period t.sub.i, the unit electrode X.sub.i of the scanning electrodes is selected by the switch 7.sub.i, and the unit electrode Y.sub.j of the data electrodes is selected by the switch 11.sub.j. Then, the potential of the unit electrode X.sub.i of the scanning electrodes is quickly set at the ground potential. However, the current from the current supply means 10.sub.i connected to the unit electrode Y.sub.j of the data electrodes through the switch 11.sub.j is used to cancel the storage capacitance in the reverse bias direction of at least (n-1) pixels described above. Hence, the potential of the unit electrode Y.sub.j of the data electrodes does not increase at once, and accordingly a delay time t.sub.d occurs until a forward bias is applied to the pixel D(i, j) to cause it to emit light. In particular, if the current supply means 10.sub.j is a constant-current circuit, the potential of the unit electrode Y.sub.j of the data electrodes increases only as a linear function of time elapsed since the unit electrode Y.sub.j is selected. As a result, the delay time t.sub.d described above increases further.